A railroad truck assembly can be utilized to measure the condition of the railroad track and the effect of adverse conditions on the trucks of the railcars. The most sensitive measurements can be made by mounting transducers directly on the wheels and axles of a railcar truck, to avoid absorption of shocks that occurs during their transmission through springs to the frame of the truck assembly. However, if transducers are to be mounted on the rotating shaft, provisions must be made to provide an interface between the transducers and circuitry on the stationary frame (which travels with the wheel but does not rotate). Since the axle is mounted on springs and is subjected to considerable shocks during high speed travel, any data interface must operate reliably despite considerable sudden movements of the axle with respect to the frame. The interface apparatus, as well as the transducers and processing circuitry to be mounted on the axle, should all be mountable without requiring disassembly of the truck assembly, and without altering it enough to appreciably affect the measurements, so that the measurements represent approximately the effects of the rails on ordinary truck assemblies. Optical techniques are well known for use in data transmission between stationary bodies, but special problems arise where one of the bodies is rotating rapidly with respect to the other.